The Antioxidant Function of Iodide in Kelp Impacts Coastal Climate

A new study found that large brown seaweeds, when under stress, release large quantities of inorganic iodine into the coastal atmosphere, where it can contribute to cloud formation, thus influencing coastal climate.

The large brown kelps of the genus Laminaria are known as the world’s most effective bio-accumulators of iodine, an important element for thyroid function in humans. The chemical speciation and biological role of iodine in kelp had, however, remained enigmatic until now.

A scientific paper which has just been published in the Proceedings of the National Academy of Sciences of the USA (PNAS) identifies that iodine is stored in the form of iodide - single, negatively charged ions - which acts as the first known inorganic (and, in fact, the most simple) antioxidant in any living system. Iodide is accumulated in the cell wall space of the outer (cortical) seaweed tissues.

“When kelps experience stress, for example when they are exposed to intense light, desiccation or atmospheric ozone during low tides, they very quickly begin to release large quantities of iodide from stores inside the tissues. Iodide ions detoxify ozone and other oxidants that could otherwise damage kelp, and, in the process, produce molecular iodine” explains lead author, Dr Frithjof Küpper from the Scottish Association for Marine Science. “Our new data provide a biological explanation why we can measure large amounts of iodine oxide in the atmosphere above kelp forests at low tide. These chemicals form particles which act as condensation nuclei around which clouds may form.”, adds Dr Gordon McFiggans from the University of Manchester. Similarly, large amounts of iodide are released from kelp tissues into sea water as a consequence to the oxidative stress during a defence response against pathogen attack.

Kelps thus play an important role in the global biogeochemical cycle of iodine and in the removal of ozone close to the Earth’s surface. This interdisciplinary and international study - with contributions from the United Kingdom, the Netherlands, Germany, France, Switzerland, the European Molecular Biology Laboratory (EMBL) and the USA - comes almost 200 years after the discovery of iodine as a novel element in kelp ashes in the context of the search for new raw materials for explosives during the Napoleonic wars. The team used a range of physical, chemical and biochemical techniques, from synchrotron-based X-ray absorption spectroscopy to gas-phase particle counters, gas chromatography-mass spectrometry (GC-MS), voltammetry to enzyme and luminescence assays and flow cytometry.

Iodide, the accumulated form of the biologically essential element iodine in Laminaria (kelp), constitutes the first identified case of an inorganic antioxidant in life, impacting atmospheric and marine chemistry. Iodide scavenges both aqueous and airborne oxidants at the surface of kelp tissues. In the case of ozone, this results in the formation of aerosol particles, contributing to coastal cloud formation.

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